1. Field of the Invention
The present invention relates to a dressing apparatus and a dressing method for dressing a polishing pad used in polishing of a substrate, such as a semiconductor wafer. More particularly, the present invention relates to a dressing apparatus and a dressing method used in a polishing apparatus for polishing the substrate to planarize a surface of the substrate. The present invention also relates to a polishing apparatus having such a dressing apparatus.
2. Description of the Related Art
Semiconductor devices become smaller and smaller in recent years, and device structures become more complicated. A surface planarization is an essential process in fabrication of the semiconductor devices. A typical technique used in the surface planarization is chemical mechanical polishing (CMP). In this chemical mechanical polishing, a substrate is brought into sliding contact with a polishing surface of a polishing pad, while a polishing liquid, containing abrasive particles such as silica (SiO2), is supplied onto the polishing surface, whereby a surface of the substrate is polished.
The chemical mechanical polishing is performed using a CMP apparatus. The CMP apparatus includes a polishing table with a polishing pad attached to an upper surface thereof, and a top ring for holding a substrate, such as a semiconductor wafer, which is a workpiece to be polished. While the polishing table and the top ring are rotated about their own axes respectively, the top ring presses the substrate against a polishing surface (i.e., an upper surface) of the polishing pad at predetermined pressure to cause sliding contact between the substrate and the polishing pad. In this state, the polishing liquid is supplied onto the polishing surface of the polishing pad. The substrate is thus polished in the presence of the polishing liquid between the substrate and the polishing pad. The surface of the substrate is planarized by a combination of a chemical polishing action by alkali and a mechanical polishing action by abrasive particles.
When the substrate is polished, the abrasive particles and polishing debris adhere to the polishing surface (the upper surface) of the polishing pad. In addition, characteristics of the polishing pad are altered and its polishing performance is lowered. Consequently, as polishing of the substrate is repeated, a polishing speed (i.e., a removal rate) is lowered and uneven polishing occurs. Thus, in order to regenerate the deteriorated polishing surface of the polishing pad, a dressing apparatus is provided adjacent to the polishing table. This dressing apparatus regenerates the polishing surface of the polishing pad by slightly scraping off the polishing surface.
FIG. 1 is a schematic view showing a conventional dressing apparatus. As shown in FIG. 1, the dressing apparatus includes a dresser disk 131, an air cylinder 136 for pressing the dresser disk 131 against a polishing pad 10, and a dresser drive shaft 132 coupling the dresser disk 131 and the air cylinder 136 to each other. The dresser drive shaft 132 is divided into a rotating section coupled to the dresser disk 131 and a non-rotating section coupled to the air cylinder 136. The rotating section and the non-rotating section are coupled to each other via a coupling 137.
The rotating section of the dresser drive shaft 132 is supported by a ball spline 135. This ball spline 135 is a linear motion guide which transmits a torque to the dresser drive shaft 132, while allowing a straight line motion of the dresser drive shaft 132 in a longitudinal direction thereof. The ball spline 135 is coupled to a motor (not shown), so that the dresser disk 131 is rotated by the motor through the dresser drive shaft 132.
The air cylinder 136 is a double-acting air cylinder in which two pressure chambers are provided on both sides of a piston 136a. Air, with adjusted pressure, is injected into each pressures chamber. Specifically, compressed air to generate a load on the polishing pad 10 is introduced into the upper pressure chamber, and on the other hand compressed air to support a weight of a movable section, including the dresser disk 131 and the dresser drive shaft 132, is introduced into the lower pressure chamber. The pressure of the air supplied to the lower pressure chamber is kept constant. A pressing force of the dresser disk 131 against the polishing pad 10 is determined by differential pressure between the upper pressure chamber and the lower pressure chamber.
Hard abrasive particles, such as diamond particles, are fixed to a lower surface of the dresser disk 131. This lower surface of the dresser disk 131 constitutes a dressing surface for conditioning the polishing surface of the polishing pad 10. When dressing the polishing pad 10, the dresser disk 131 is pressed against the polishing pad 10, while a polishing table 11 and the dresser disk 131 are rotated and pure water is supplied onto the polishing surface of the polishing pad 10. The polishing surface of the polishing pad 10 is dressed (or conditioned) by sliding contact between the dressing surface of the dresser disk 131 and the polishing surface.
During dressing, the polishing surface of the polishing pad 10 is scraped by the dresser disk 131. The pressing force of the dresser disk 131 against the polishing pad 10 has a great influence on a life of the polishing pad 10. Therefore, it is necessary to accurately control the pressing force of the dresser disk 131. In the above-described structures, since the air having constant pressure is supplied into the lower pressure chamber of the air cylinder 136, the pressing force of the dresser disk 131 depends on the pressure of the air introduced into the upper pressure chamber. Thus, calibration is necessary in order to establish a relationship between the pressing force of the dresser disk 131 and the pressure of the air introduced into the upper pressure chamber of the air cylinder 136.
The calibration is performed by inserting a load-measuring device (e.g., a load cell) between the polishing pad 10 and the dresser disk 131 and associating measurement value (i.e., the pressing force), obtained from the load-measuring device, with the pressure of the air supplied to the air cylinder 136. However, in order to carry out the calibration, it is necessary to stop operations of the polishing apparatus. As a result an operation rate of the polishing apparatus is lowered.
In addition to the above-described problem, the dressing apparatus using the air cylinder entails the following drawback. As described above, the pressing force of the dresser disk 131 against the polishing pad 10 affects the lifetime of the polishing pad 10. Therefore, in order to extend the life of the polishing pad 10, it is necessary to decrease the pressing force of the dresser disk 131 to some extent. However, when the pressure of the air in the upper pressure chamber of the air cylinder 136 is lowered, the piston may not move in spite of the differential pressure between the upper pressure chamber and the lower pressure chamber. This is because, when the differential pressure between the upper pressure chamber and the lower pressure chamber is close to zero, frictional resistance between the piston and a cylinder and frictional resistance between the dresser drive shaft 132 and the air cylinder 136 become relatively high. In such a dead zone where the air cylinder 136 does not operate, good dressing of the polishing pad 10 is not performed and as a result, stable polishing performance of the polishing pad 10 cannot be achieved.